1. Field of the Invention
The present invention relates to the field of transmission technology and a (continuously variable) hydrostatically power-splitting transmission.
2. Discussion of Related Art
Power-splitting transmissions, particularly for employment in vehicles of agricultural or building use, such as, for example, tractors, have been known for a long time. In such power-splitting transmissions, the power prevailing at an input shaft or drive shaft and usually output by an internal combustion engine is apportioned to a first mechanical power branch with a fixed step-up ratio and a second power branch having a continuously variable step-up ratio and is subsequently combined again in order to be available at an output shaft or take-off shaft. The second power branch is mostly designed as a hydrostatic branch in which two hydrostatic axial piston engines (hydrostats) of the oblique axis or swashplate type, which are connected to one another hydraulically, operate selectively as a pump or as a motor. The step-up ratio can in this case be varied by a variation of the pivot angle of the cylinder block or the swashplate. The apportionment of the power to the two power branches and the combining of the split powers usually take place by means of an epicyclic transmission. Power-splitting transmissions of the type described are disclosed in various configurations in DE-A1 27 57 300, in DE-C2-29 04 572, in DE-A1-29 50 619, in DE-A1-37 07 382, in DE-A1-37-26 080, in DE-A1-39 12 369, in DE-A1-39 12 386, in DE-A1-43 43 401, in DE-A1-43 43 402, in EP-B1-0 249 001 and in EP-A2-1 273 828.
So that a power-splitting transmission can be successfully employed in practice, it should, in general, be distinguished by the following properties:                The transmission should have high efficiency over the entire speed range. This should be the case particularly at the high driving speeds which are adopted in road traffic for a relatively long period of time.        The transmission should have a compact construction in order to allow installation in the most diverse possible vehicles, as far as possible without structural restrictions.        The transmission should allow the transfer of high powers.        The transmission should have as simple a construction as possible in order to limit the power losses and increase the operating reliability.        The transmission should allow fully comprehensive electronic control in conjunction with the engine management and should make available sufficient emergency running programs even in the event of a failure of specific control elements.        
The initially mentioned DE-A1-43 43 402 has already described a power-splitting transmission, designated as a CHP transmission (Continuously variable Hydrostatic Power-splitting transmission), which is distinguished by two hydraulically coupled identical hydrostats of the oblique axis type of construction, which can be coupled in different ways to an epicyclic differential transmission via pairs of clutches or change-shift elements K1/K2 or K3/K4. The known CHP transmission has been employed and tested under the type designation SHL-Z in city buses. The two hydrostats employed have a pivoting range of only 0-25°. For forward travel, in this case, three driving steps or driving ranges are obtained: in the first driving range, at the starting point the hydrostatic fraction of the transferred power is 100% and then moves linearly with the speed toward zero. In the second driving range, it moves from zero to a maximum of about 27% and then back again to zero. In the third driving range, it moves from zero to a maximum value of 13% at the highest forward speed.
The hydrostatic power transfer branch of such a transmission usually comprises two hydrostatic axial piston engines which are connected hydraulically to one another and of which in each case one operates as a pump and the other as a motor. Depending on the driving step, in this case, the two engines can interchange their roles.
The hydrostatic axial piston engines constitute an essential component of the hydrostatic power-splitting transmission and decisively affect the properties of the transmission, such as, for example, the efficiency, overall size, complexity, speed range covered, type and number of driving steps, and the like. Examples of hydrostatic axial piston engines of this type are disclosed in DE-A1-198 33 711 or in DE-A1-100 44 784 or in US-A1-2004/0173089. The functioning and theory of hydrostatic axial piston engines and of a power-splitting tractor transmission equipped with them are described in a publication of TU Munich from the year 2000 by H. Bork et al., “Modellbildung, Simulation and Analyse eines stufenlosen leistungsverzweigten Traktorgetriebes [Modelling, Simulation and Analysis of a continuously variable power-splitting tractor transmission]”.
In the known hydrostatic transmissions, the parts in the transmission (hydrostats, clutches, shafts, epicyclic drives, gearwheels, etc.) are installed in a housing which is oriented specially with respect to the transmission and consists of a multiplicity of housing segments. If, then, such a transmission is to be installed in a corresponding agricultural or building vehicle, either the vehicle has to be coordinated in its design with the already prefabricated transmission or the transmission has to be coordinated with the given conditions of an already existing vehicle and therefore redesigned. In both instances, a considerable extra outlay arises due to the special adaptation of the vehicle or entire transmission.
In the publication DE-A1-26 33 718, it has already been proposed to construct a simple hydrostatic transmission without power splitting so that it forms a structural unit with the cover of the transmission housing. In the transmission housing itself, only the drive and take-off shafts accessible from outside are mounted, and come into engagement via internal gearwheels with the corresponding inputs and outputs of the transmission when the cover together with the transmission is placed on the transmission housing.
What is achieved thereby is that the housing together with the drive and take-off shafts can be installed in the vehicle at an early stage, while a decision can be made later, by a cover together with a corresponding transmission unit being put in place, as to whether a mechanical or a hydrostatic transmission is to be used. Correspondingly, transmissions can be exchanged in a simple way in the already finished vehicle.
The transmission concept (structural unit of transmission and cover) known from DE-A1-26 33 718 may be useful for the simple case of a transmission without power splitting, when neither clutches nor summing members are required and only one of the hydrostats is adjusted. It is sufficient here to arrange the adjusting mechanism for the one hydrostat directly on the hydrostat inside the housing.
For the substantially more demanding concept of a continuously variable hydrostatic power-splitting transmission, however, other ways must be found not only to accommodate the markedly more complicated control, but also to place it suitably in terms of assembly and of maintenance.